1. Field of the Invention
This invention relates to improvements in apparatus for microanalysis, by utilizing a charged particle beam as a probe such as an electron beam or an ion beam, and more particularly to a scanning electron microscope which is provided with a control circuit for adjusting the contrast and brightness of an image or a picture representative of a surface layer of an object to be observed.
2. Description of the Prior Art
For the purpose of microanalysis, an apparatus utilizing a charged particle beam, such as an X-ray microanalyzer, Auger electron microanalyzer and scanning electron microscope is frequently used. This invention is applicable to all the abovementioned apparatus, however, for the sake of explanatory convenience, a description will be made hereafter in terms of the scanning electron microscope.
With the scanning electron microscope, a surface layer of an object to be examined is irradiated and two-dimensionally scanned with an electron beam focussed at a microregion on the object. Secondary electrons thus emitted from the surface layer are detected and amplified by means of a photo-multiplier and then applied to a control grid of a cathode ray tube so that an electron beam irradiating a fluorescent screen can be modulated.
When both the electron beams irradiating the object and the fluorescent screen are two-dimensionally scanned in synchronism, the view or condition of the surface layer can be observed as an enlarged image or picture on the fluorescent screen.
In order to obtain the best quality and the emphasized contrast of the enlarged image, a video signal applied to the control grid of the cathode ray tube is required to have a D.C. level and a maximum amplitude just within the operating range of the cathode ray tube. That is, the video signal, D.C. level which corresponds to the brightness in the darkest portion on the object, is selected by adjusting the bias voltage of the video amplifier, to have a value corresponding to an electron beam intensity equal to or slightly higher than an intensity at a rising part of the brightness-beam intensity characteristic of the fluorescent screen.
Also, the maximum amplitude of the video signal, which represents brightness in the brightest portion of the object, is adjusted by controlling the gain of the video signal to a value corresponding to an electron beam intensity being equal to or slightly lower than the value at the beginning of a saturating part of the brightness-beam intensity characteristic.
These adjustments are very troublesome and time consuming since they are frequently required at, for example, a time when it is necessary to change the intensity of an electron beam irradiating an object or when it is necessary to examine various kinds of objects.
To eliminate such disadvantages, one type of apparatus has been proposed in which a detector is provided for detecting the minimum and maximum values of a video signal to be applied to a control grid of a cathode ray tube. In response to the thus detected signals, the bias voltage and the gain of a video amplifier are automatically controlled so as to maintain the D.C. level and the peak value of the video signal at predetermined levels.
However, this improved apparatus still has disadvantages such as deteriorated quality of the image or the picture showing the object because of fluctuations or changes in the beam emission characteristics and the operating voltage of the cathode ray tube. That is, even though the peak value of the video signal is fixed at a predetermined value, the electron beam intensity often exceeds the value at which the brightness of the image or the picture is saturated if the electron emission efficiency from a gun is increased. In this case, the brightest portion of the object cannot be observed in detail.
On the other hand, when the emission efficiency is decreased, the intensity of the electron beam is often less than the value at a rising part of the brightness-beam intensity characteristic of the fluorescent screen, so that the darkest portion of the object cannot be observed in detail.